1. Field of the Invention
The invention relates to a regenerative shaft furnace for burning carbonate-containing raw materials such as limestone, dolomite, magnesite, etc. More particularly, the invention pertains to such furnaces which have two or more shafts interconnected by ducts and, in an alternating manner, one shaft operates as the burning or parallel flow shaft and the other operates as the counterflow shaft, with the burned raw material being cooled in the cooling zone of the shafts.
2. Description of the Prior Art
Furnaces having two or more shafts are used for burning carbonate-containing raw materials for the purpose of expelling the carbon dioxide as disclosed in Austrian Pat. No. 211,214 and the Journal "Zement-Kalk-Gips" No. 6, 1980, pp. 217ff. They have been very satisfactorily used for many years, because they have a low heat consumption and the burned material produced is of very good quality. The shafts of such a furnace are alternately operated as a parallel flow shaft or a counterflow shaft for the purpose of removing the flue gases. In each shaft, there is a preheating zone, a burning zone, and a cooling zone extending in this order from the upper edge of the shaft to the furnace foundation. In operation, the fuel and the combustion air is supplied to only one shaft, namely the parallel flow or burning shaft, but the cooling air or medium is continuously removed by means of a removal mechanism.
In the case of furnaces having large cross-sections which is often due to constructional reasons, it may happen that the CO.sub.2 -containing flue gases enter the cooling zones at a low temperature. In the case of burned quicklime (CaO), the CO.sub.2 is reabsorbed by the quicklime. This leads to a deterioration of the quality of the lime burned in the burning zone and this quality cannot be improved by supplying more fuel at the beginning of the burning zone. Moreover, since the quantity of reabsorbed carbon dioxide is not generally known, such as increased fuel supply can lead to a large amount of heat in the burning zone and consequently to an undesired, harder lime quality and even to block formation. The quantity of reabsorbed carbon dioxide in the burned lime can vary between a few tenths of a percent and 1 to 2 percent. Reabsorbed carbon dioxide is particularly undesirable when burning lime or dolomite for producing sea water magnasite. In this production process, particularly strict requirements are placed on the residual CO content of the burned material, which must be below 0.5% and even below 0.3%.